Acousto-optic devices of conventional design, exploiting the effects of Bragg diffraction in particular, have until recently required excessive levels of acoustic power to obtain high efficiency, particularly in the infra-red region of the spectrum. However, techniques for radically reducing the velocity of propagation of light through solid media have recently been developed with the advantage that the efficiency of acousto-optic interactions within the media is greatly enhanced allowing for significantly lower levels of acoustic power to implement these devices. These techniques depend on the creation of distributed optical resonators inside a dielectric medium which produce strong dispersion of refractive index around their resonance frequency, and hence a low group velocity for light having a similar frequency passing through the medium. Examples of such media include photonic crystals in which the group velocity of light is reduced to a value at least 100 times smaller than the velocity of light in a vacuum. Materials having this property will be referred to as “slow light” materials in the present patent specification. Such materials are being applied in novel ways to make improved acousto-optic devices of various types.